1. Field of the Invention
This invention relates to a photographing device of the kind performing exposure control with a plurality of blades.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings show, in an exploded oblique view and a plan view, the arrangement of the essential parts of the conventional photographing device. The device is of the kind having a plurality of blades with an exposing aperture hole formed in each of the blades and is arranged to control the amount of exposure of a light receiving part such as a film, a CCD or the like by driving these blades.
Referring to FIGS. 1 and 2, a motor 1 is arranged to perform a shutter opening action and is secured to a base plate 21. A pinion gear 2 is fitted on the shaft of the motor 1 with pressure and is arranged to cause a rack gear 5 to slide along shafts 21g and 21f provided on the base plate 21 through first and second reduction gears 3 and 4 which are fitted on gear shafts 21a and 21b provided on the base plate 21. A clutch 7 and a clutch spring 8 are mounted on the rack gear 5 by means of a shaft 9. The clutch spring 8 has a spring piece 8a which engages the clutch 7 and is arranged to urge the clutch 7 to turn clockwise, i.e., in the direction of arrow A as shown in FIG. 1.
A shutter plate 10 is arranged to turn around a hole 10a in the direction of arrow B as shown in FIG. 1 when a projection 10b provided on the shutter plate 10 is pushed by the end part 7a of the clutch 7. A gear 10c which is formed along the peripheral part of the shutter plate 10 is arranged to drive the gear part 11a of a pulse plate 11. The pulse plate 11 is provided with a return spring 12 and a photointerrupter 13 which is arranged to read the bright-and-dark pattern formed on the pulse plate 11.
Two shutter blades 14 and 15 (hereinafter referred to as blades) respectively have exposing aperture holes 14a and 15a (hereinafter referred to as aperture holes) which are formed to define an aperture, and include exposing aperture parts 14c and 15c for high luminance (hereinafter referred to as high luminance aperture parts), and slots 14b and 15b which are arranged to be driven by projections 10d and 10e provided on the shutter plate 10 as shown in FIG. 2 and to be guided by shafts 21c and 21d provided on the base plate 21.
As shown in FIG. 3(a), the aperture holes 14a and 15a which are formed in the blades 14 and 15 to include the high luminance aperture parts 14c and 15c are in the same shape and symmetrically arranged relative to an optical axis La.
The positions of the slots 14b and 15b of the blades 14 and 15 in relation to the shafts 10d and 10e of the shutter plate 10 are arranged to be such that, when the blades 14 and 15 are moved in parallel to each other by the rotation of the shutter plate 10, an aperture is formed by the high luminance aperture parts 14c and 15c in a shape which is symmetrical relative to the optical axis La as shown in FIG. 3(b).
With the blades 14 and 15 moved in parallel by the rotation of the shutter plate 10 at the time of high luminance shooting, for example, the center points 14d and 15d of the high luminance aperture parts 14c and 15c (hereinafter referred to as center points) simultaneously come to traverse the optical axis La of a photo-taking lens (hereinafter referred to as the optical axis) from the right and left sides of the optical axis La, respectively.
The center points 14d and 15d which thus simultaneously come to traverse the optical axis La are defined to be in phase relative to the optical axis La. Further, the fore end point e of the high luminance aperture part 14c traverses the optical axis La earlier than the center points 14d and 15d when the shutter plate 10 rotates. The phase of the fore end point e is therefore defined to be in advance of the center points 14d and 15d relative to the optical axis La.
A suction type magnet M is provided with an armature 16. When a current is applied, the armature 16 is sucked or attracted upward. A bent piece 16a of the armature 16 then comes to hit a projection 7b provided on the clutch 7 to disengage the end part 7a of the clutch 7 from the projection 10b of the shutter plate 10. A reference numeral 17 denotes the armature shaft of the magnet M. A numeral 18 denotes a yoke. A numeral 19 denotes a coil. A numeral 20 denotes a coil shaft. A reference symbol S denotes a light receiving surface.
The operation of the conventional device is described below with reference to FIGS. 1 and 2:
When the shutter release switch of a camera which is not shown is pushed, the motor 1 is driven. The rack gear 5 is then driven through the pinion 2, the first and second reduction gears 3 and 4 to move in the direction of arrow C along the shafts 21g and 21f of the base plate 21. The clutch 7 mounted to the rack gear 5 is urged by the clutch spring 8 in the direction of arrow A. With the rack gear 5 thus driven, the end part 7a of the clutch 7 engages the projection 10b of the shutter plate 10 to cause the shutter plate 10 to turn in the direction of arrow B while following the movement of the rack gear 5.
The peripheral gear 10c of the shutter plate 10 then causes the pulse plate 11 to turn in the direction of arrow F as shown in FIG. 2 against the urging force of the spring 12. With the pulse plate 11 thus turned, the photointerrupter 13 generates pulses according to the angle of rotation of the shutter plate 10. Then, at the same time, the projections 10d and 10e of the shutter plate 10 drive the blades 14 and 15 by engaging their slots 14b and 15b, respectively. Therefore, an exposure is effected with an aperture defined jointly by the aperture holes 14a and 15a which respectively include the high luminance aperture parts 14c and 15c.
The number of pulses generated by the photointerrupter 13 increases and the aperture holes 14a and 15a move to gradually open an exposing aperture D accordingly as the shutter plate 10 turns. When the aperture D reaches a shape corresponding to the measured brightness of an object to be photographed, a current is applied to the coil 19 of the magnet M to attract the armature 16. With the armature 16 thus attracted, the bent piece 16a of the armature 16 hits the projection 7b of the clutch 7 to disengage the end part 7a of the clutch 7 from the projection 10b of the shutter plate 10 against the force of the clutch spring 8.
The return spring 12 then acts to cause the shutter plate 10 to begin to quickly turn clockwise in the returning direction which is reverse to the direction of arrow B as shown in FIG. 1. The aperture holes 14a and 15a of the blades 14 and 15 are closed. After that, the power supply to the coil 19 of the magnet M is cut off. After that, a current is reversely applied to the motor 1. The motor 1 then causes, via the pinion 2, the first reduction gear 3 and the second reduction gear 4, the rack gear 5 to slide back in the direction reverse to the direction of arrow C of FIG. 1. The end part 7a of the clutch 7 on the rack gear 5 then engages again the projection 10b of the shutter plate 10 to bring the device back to a state obtained before shooting.
With the conventional device arranged in the manner as described above, the two blades 14 and 15 are disposed before and behind in the direction of the optical axis La. FIG. 4 shows the arrangement of exposure effecting parts around the optical axis in a sectional view taken on a line E--E in FIG. 2.
Referring to FIG. 4, the blades 14 and 15 are disposed within a space l between the base plate 21 and a plate 22 (which is not shown in FIG. 1). To smoothen and stabilize the movement of the blades 14 and 15, the space l is arranged to leave some margins for the thicknesses of the blades 14 and 15. Therefore, the blades 14 and 15 are allowed to have some clearance between them as shown in FIG. 4.
In this instance, the shape of the aperture D is symmetric relative to the optical axis La. Therefore, while light fluxes in parallel to the optical axis La are equally incident on the right and left sides of the optical axis La, the quantities of oblique incident light fluxes R and L which are oblique relative to the optical axis La become not equal to each other. The uneven light quantities present the following problem:
In taking a shot of an object with a background of uniform luminance, such as a white wall, a scenery covered with snow or the like, the brightness of the shot taken becomes uneven between the right and left peripheral parts. This uneven brightness results from the fact that a plurality of blades having the aperture holes are disposed in different positions before and after each other in the direction of the optical axis.
Further, the adverse effect of the unevenness of the oblique incident light fluxes L and R tends to be great when the aperture is small, that is, when the luminance of the photographed object is high.
The size of the aperture decreases accordingly as the size of the light receiving part decreases. The size of the light receiving part is smaller when it is a CCD (which is a known kind of photoelectric conversion means) than when it is a film and also decreases when the size of the CCD changes from 2/3 inch to 1/3 inch and the adverse effect of the unevenness of oblique incident light fluxes also increases accordingly. Therefore, in the case of an electronic still camera or a video camera using a CCD for the light receiving part, the adverse effect of the unevenness of oblique incident light cannot be ignored.
Further, since reduction in size of the CCD for reduction in size of the above-stated apparatus has influence on the whole photo-taking lens system, it is very important to eliminate the adverse effect of the unevenness of the oblique incident light.